Every year, millions of Americans are diagnosed with lower back pain and, in up to 22% of cases, the causes of the pain originate from the sacroiliac joint. For cases of degenerative sacroiliitis, inflammatory sacroiliitis, iatrogenic instability of the sacroiliac joint, osteitis condesans ilii (OCI), and traumatic fracture dislocation of the pelvis, surgeons commonly employ stabilization and/or fusion at the sacroiliac joint; however, a growing body of research suggests that sacroiliac joint fusion may also be effective in treating lower back pain. The sacroiliac joints are diarthrodial synovial joints, connecting the sacrum to the ilium, that facilitate transmission of forces between the spine and the lower extremities; there is one sacroiliac joint on the left side of the sacrum and one sacroiliac joint on the right. The sacrum and the ilium are particularly coarsely textured with several ridges and depressions throughout both respective surfaces.
Additionally, the sacroiliac joint contains a very narrow joint space between the ilium and the sacrum. Despite its relatively small size, the biomechanics of the sacroiliac joint are complex, with movement occurring both in the transverse and the sagittal planes. Stability between the two surfaces is maintained by a combination of strong sacroiliac, sacrotuberous and sacrospinous ligaments, bony interdigitation, and several muscles that act directly upon the region. Due to the sacroiliac joint's anatomical location, examinations and research can often be difficult. However, with modern medical imaging systems such as computer tomography, magnetic resonance imaging, and fluoroscopic guidance, understanding and treating disorders at this joint are becoming increasingly effective.